When I visualize the IMU simulation using ROS RViz IMU plugin, the linear acceleration vector is directing downward (toward the ground), while this should direct upward (toward the sky) as shown in the following URL:
http://wiki.ros.org/rviz_imu_plugin?action=AttachFile&do=get&target=rviz_imu_plugin.png

This is probably because the gravity vector is added in the UnitySensors implementation:
https://github.com/Field-Robotics-Japan/UnitySensors/blob/master/Assets/UnitySensors/Runtime/Scripts/Sensors/IMU/IMUSensor.cs#L51

While the gravity vector is subtracted in the other implemenation (e.g. gazebo):
https://github.com/gazebosim/gazebo-classic/blob/gazebo11/gazebo/sensors/ImuSensor.cc#L375-L376

We probably need to think "Because the IMU is not moving, linear velocity is applied against the gravitational force (that is why the vector should be subtracted)".